I have a theory about this which I'm trying to get Apple to confirm or deny; in the meantime, here it is.

When Microsoft first told me about its plan to launch subscription DRM (then called "Janus") that is capable of expiring content on devices, Microsoft staff at the meeting explained that the linchpin of the system is a "secure digital clock" inside certain supported portable media players, which cannot be altered by software, and which the DRM system uses to determine whether a user still has the rights to play a file. (Did the user pay their monthly subscription fee? Has a movie expired?)

I have long maintained that Apple would have a hard time launching amusic subscription service, not only because Steve Jobs thinks they'rethe wrong approach, but also because legacy iPod models, lacking a secure digital clock, would beincapable of playing subscription files. IfApple wanted to launch a music subscription, it would have to releasenew devices with such hardware included.

That appears to be exactly what has happened here with iTunes MovieRentals. Jobs has said for years that renting movies makes sense whereas renting music doesn't. When it came time to launch a movie rental service, Apple had to startincluding secure digital clocks in the latest round of devices. Connected devices like the iPhone have other ways of checking content permissions, but for such a service to work with iPods, the rental system would require something resembling Microsoft's solution, with a secure digital clock, unless there's something I'm missing (granted, I'm a tech journalist, not an engineer). My theory is that only the latest iPods have the clock, which is why previous models can't support movie rentals.

Apple is notoriously tight-lipped about this sort of thing, but I havea question in with a contact there, and am hoping they'll confirm mytheory.